The prior art has recognized the need for thermostatically controlled hydraulic drain valves for draining water systems in buildings and equipment to prevent breakage during freezing. These drain valves were generally powered by spring-trip mechanisms or expandable bellows mechanisms. The usefullness of bimetallic elements in thermostatics has been recognized, typically requiring a source of electrical power to couple the thermostatic to a separately powered valve. Only the crudest efforts have been made to power the valve directly by the action of the bimetallic element.